AstraZeneca has "voluntarily paused" the late-stage trials of the long-awaited COVID-19 vaccine it is developing due to safety concerns.

In partnership with the University of Oxford, AstraZeneca worked with volunteers in developing the COVID-19 vaccine. The company had to halt production because one volunteer developed an unexplained illness, AstraZeneca said Wednesday.

The COVID-19 vaccine by AstraZeneca is now at its third phase. It has dozens of sites across the U.S.

The participant who suffered from an adverse reaction to the vaccine is one in the United Kingdom, reported Stat News.

It is not yet clear what the adverse reaction was. When it happened, it is also unknown. But the volunteer is likely to recover, according to a source familiar with the matter.

AstraZeneca Chose to be Cautious

A spokesperson for the company spoke about the concern. They said their review process was triggered as part of their ongoing trials on the vaccine.

The company said they need to review safety data through an independent committee.

They said the pause was only a "routine action" that had to occur when a possibly unexplained illness happens in one of the trials. They also ensured that they are maintaining the integrity of the tests.

In a follow-up statement, AstraZeneca also said they were the ones to initiate the hold on the Phase 3 trials. The review, which they said was a "single event," is expected to finish quickly.

They added that they were hoping to "minimize any potential impact on the trial timeline."

USA Today noted that the adverse reaction might not be linked to the vaccine at all. It is possible that pausing the trial is just an act of caution.

Dr. William Schaffner, a professor at Vanderbilt University in Nashville, Tennessee, said the company has to figure out the cause. "It's very possible that according to their protocol, as soon as the event occurs, the company can put a hold on the trial," he said.

He also noted that the group that will review the data is the Data Safety Monitoring Board.

The COVID-19 vaccine is seen as one of the most promising ones yet, noted Al Jazeera. It is under trial in countries like the U.S. and Brazil.

AstraZeneca Shares Dropped

Forbes reported that the shares for AstraZeneca went down 6% after the news.

It's not yet clear what side effect triggered the hold. But a July study stated that during the drug's Phase 1/2 said 60% of the 1,000 volunteers experienced mild to moderate side effects.

These side effects include fever, headaches, muscle pain, and injection-side reactions.

The Phase 3 trial was designed for 30,000 people across the U.S.

With the pause, AstraZeneca will be putting the recruitment of participants and dosing current ones on hold. They will return to the regular flow of the trials once it's deemed safe to dose.

In May, the U.S. government gave a $1 billion funding to the company to produce the vaccine. There are 400 million pre-ordered doses of the unproven drug for delivery.

There are other news of other trials that were on hold. This makes the AstraZeneca trial the first one to have been paused due to a possible safety threat.

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